Method and device for ensuring the run of the web in the multi-cylinder dryer of a papermachine

ABSTRACT

The multi-cylinder dryer of a papermachine comprises a first tier of cylinders, wherein a web travels supported by a first dryer wire on the cylinders of the first tier of cylinders, as well as a second tier of cylinders, wherein the web travels supported by a second dryer wire on the cylinders of the second tier of cylinders. Upon passing around its wire guide roll, each dryer wire forms a loop having at least one section, along which the first or second wire, respectively, is free from the web and which in this wire traveling direction is immediately upstream of a point, at which the first or second wire is received on the first or second guide roll. Along a wire section between a cylinder and the guide roll, a nozzle inside the respective wire loop is used blowing air through the free section of dryer wire into a pocket between it and a cylinder of the opposite tier of cylinders for ventilating the pocket through a plenum zone formed between an air guiding surface located inside the respective wire loop downstream of the nozzle in the wire traveling direction, and the free section.

BACKGROUND OF THE INVENTION

The present invention relates to a method for ensuring the run of theweb in the multi-cylinder dryer of a papermachine. The invention relatesalso to a device for carrying out the above method.

The multi-cylinder dryers of a papermachine have employed a so-calledtwin-wire run at the downstream end of a drying section. In thetwin-wire run, the wire is supported against the jackets of cylindersincluded in two tiers of cylinders by means of two wires, one runningalong a tortuous path against the jackets of cylinders of an upper tierand the other against the jackets of cylinders of a lower tier. Whenpassing over from one tier of cylinders to the other, the web travelsunsupported. At machine speeds of more than 800 m/min, the air currentsproduced by the web and moving parts of the machine cause fluttering ofthe web in these open spaces. The fluttering leads to web breakups atthe upstream end of such cylinder arrays, as the strengthcharacteristics of the web are still poor due to a high water content.

Efforts have been made to resolve this problem by using a single-wirerun, wherein the open runs of a web are eliminated and the web travelssupported all the time by one and the same dryer wire between cylindersincluded in two tiers. The drying effect of those single-tier cylinders,whereat the wire at this point lies between the web and the cylinder, isnegligible as the wire prevents the transfer of heat from the cylinderto the web. Indeed, in the most recent machines, such cylinders havebeen replaced with suction or vacuum rolls, and this has resulted inimproved machine operating characteristics and the threading ropes havebecome unnecessary.

Originally, the single-wire groups generally used to comprise just twoor three upstream drive groups of a machine, but their number has beenincreased as the machine speeds have increased. Some recent machineslack completely the twin-wire cylinder groups. A drawback in asingle-wire run is the increased length of a dryer section, leading tothe increased length of a machine hall and, thus, to the increasedfactory building costs. On the other hand, a drawback affecting thepaper quality is that, in a single-wire run, heat is always supplied tothe web from the same side of paper, resulting in possible defects inpaper (curling).

As a summary of the above alternatives, it can be said that the benefitsof a twin-wire run include two-sided drying operation and a short dryersection, but the drawbacks include poorer running characteristics athigh machine speeds. The advantages and disadvantages of a single-wirerun are essentially opposite relative to the above.

In addition, for example U.S. Pat. No. 3,753,298 discloses a machineconfiguration, which employs a twin-wire run but in which the web is allthe time supported by either one of the dryer wires. This is achieved bypassing the dryer wires by way of guide rolls mounted between the dryercylinders in such a manner that, during the passage between a dryercylinder and a guide roll, the dryer wire always runs tangentially tothe other guide roll and the other dryer wire wrapping there around,whereby the web can be transferred from one dryer wire to another atthese points without open draws. A weakness of the solution disclosed inthe cited publication is that the run of a web against a dryer wirebetween dryer cylinders and wire guide rolls is not secure. Thus, thepressure differences prevailing in pockets defined by dryer cylindersand wire sections, the air currents produced thereby and, on the otherhand, the adhesion forces between the web and cylinder surfaces detachthe web from the dryer wires. Thus, the open, unsupported web is againsusceptible to wrinkling and, at sufficiently high running speeds, thisagain leads to web breakups.

Naturally, attempts have been made to improve the operatingcharacteristics of such a machine configuration by providingmulti-cylinder dryers with air current controlling and/or producingstructure at suitable locations. This type of solutions have beendisclosed for example in Finnish Patents 68279 (Patent Application841167) and 76142 (Patent Application 854494). The passage of the webagainst the dryer wire is secured by using vacuum developing blow boxes.However, the blow box assemblies and nozzle designs proposed in theabove references require very large overall air quantities for a desiredeffect. These air quantities are typically about 2000-2400 m³ /h perpocket. In terms of energy efficiency, this is undesired and leads tovery large diameters in compensation air manifolds as well as highlycomplicated and expensive air circulation systems.

SUMMARY OF THE INVENTION

An object of the invention is to provide a method and a device foreffecting the above type of closed twin-wire run with a web supported atall times in a manner that the amount of air can be minimized. Accordingto a method of the invention, is an air guiding surface located on theother side of the dryer wire and opposite to the pocket allows to blowair from the nozzle into the pocket effectively and without largeamounts of air. On the other hand, a device for carrying out the methodincludes a guiding surface mounted on the other side of the dryer wirein conjuction with the nozzle and opposite to the pocket for guiding anair current blown from the nozzle through the dryer wire and into thepocket.

BRIEF DESCRIPTION OF THE DRAWING

The invention will now be described in more detail with reference madeto the accompanying drawing, which shows a side view of a multi-cylinderdryer included in a papermachine and provided with a device forutilizing the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

As for its general configuration, a multi-cylinder dryer shown in thedrawings is similar to that disclosed in the above-cited U.S. and FIPatents. The multi-cylinder dryer comprises cylinders 1 included in afirst tier of cylinders and cylinders 2 included in a second tier ofcylinders, these being heated cylinders intended for removing water froma web W to be dried, and traveling in close conformity therewith underthe guidance of dryer wires. A first dryer wire k1 travels in a tortuouspath around top cylinders making up the first tier of cylinders. The webW travels supported by the wire k1 and always pressed against the jacketof each cylinder 1. Between the cylinders, the wire is passed aroundguide rolls h1 located below the cylinders, the position of these rollsbeing such that the loops formed by the wire and the web pass around therolls obliquely relative to the longitudinal direction of the first tierof cylinders. In a corresponding fashion, the web W is passed around thejackets of cylinders 2, in this case the bottom cylinders, included inthe second tier of cylinders, supported by a second dryer wire k2, thiswire being passed around guide rolls h2 included in the second tier ofcylinders such that the loops formed by the wire at this point aredirected obliquely relative to the longitudinal direction of the secondtier of cylinders in the same direction as the loops formed by the firstdryer wire k1. In order to support web W over the entire distance, theloops formed by different dryer wires k1, k2 are arranged to runtangentially to each other such that over the section, wherein seconddryer wire k2 travels from cylinder 2 to guide roll h2, it will runtangentially to the guide roll h1 of first dryer wire k1, whereby theweb W extending from the jacket of the cylinder 2 can be transferred tothe loop of wire k1 running around guide roll h1 for carrying it furtherto the jacket of first cylinder 1. Since a wire loop located downstreamof this cylinder in the web traveling direction runs in turntangentially to guide roll h2 of that wire loop from which the web wastransferred, it can be transferred again back to second dryer wire k2for carrying it to a second-tier cylinger 2. In order to secure thetransfer, the wire guide rolls h1 and h2 are suction rolls.

The first dryer cylinder 1, a section of first dryer wire k1 extendingfrom guide roll h1 to this cylinder, as well as a wire section leavingcylinder 1 and located between the cylinder and the guide roll h2 ofsecond dryer wire k2 define together with the second dryer wire k2 aclosed pocket t1,2. A corresponding pocket t2,1 is formed at the secondcylinder 2.

The invention will now be described with reference made to equipmentassociated with the loop formed by the second dryer wire k2 winding onguide roll h2, but the same applies analogically also to equipmentassociated with the loop of the first dryer wire k1. Both of these loopsinclude three sections having identified functions and being thereforedesignated with identical reference signs in both loops: a section a-c,wherein the wire runs along with web W from the cylinder to the locationof wire guide roll, at which the wed is transferred to the dryer wirepassing therearound, a section c-d, wherein the wire starting from thetransfer point c progresses to the wire guide roll, as well as a sectione-f, wherein the wire carrying the web W transferred back travels fromguide roll to dryer cylinder.

At the loop formed by second dryer wire k2, along a section a-d betweendryer cylinder 2 and guide roll h2, is mounted a nozzle s2 is mounted,located on the opposite side of wire k2 relative to pocket t1,2. Thenozzle is intended for blowing air into pocket t1,2 for its ventilationand for securing the stability of web W over the sections of first dryerwire k1 defining the pocket, the nozzle having the same basic purpose asthe nozzles disclosed in the above-cited Finnish Patents 68279 and76142. Downstream in the traveling direction of dryer wire k2 the nozzles2 is followed by an air guiding surface op, facing a wire section c-d.The air blown from nozzle s2 is guided along this section into a spacebetween wire k2 and surface op, which space will thus beover-pressurized and this plenum zone is indicated with referencecharacter pl+. The air guiding surface, which extends roughly parallelto section c-d, covers most of the length of this section, extendingpreferably to the proximity of the jacket of wire guide roll h2. In thedrawing, the surface extends all the way to the gap between the wire andthe roll, i.e. it extends beyond a plane tangential to the roll jacketand is perpendicular to wire k2. Thus, the gap n+ between dryer wire k2and roll h2 located upstream of point d will also be over-pressurized.In view of intensifying the gap over-pressure, a sealing t is fittedbetween the jacket of guide roll h2 and an arched surface hp facing theroll jacket and adjoining the end of air guiding surface op and the rolljacket. The blowing air travels from plenum space p1+, n+ limited bywire k2 into pocket t1,2 through the air-permeable section c-d of dryerwire k2 not covered by web W, taking care of ventilation of the pocket.

The nozzle s2 is located in the traveling direction of dryer wire k2upstream of point c, at which web W is transferred from second dryerwire k2 to first dryer wire k1. The location, at which the plenum zonebetween the dryer wire and the guiding surface begins, is indicated withreference character b and it is located upstream of point c. In thedryer wire traveling direction, upstream of point a, at which the dryerwire departs from cylinder 2, a nozzle s1 is located for producing anair blow directed against the wire traveling direction. Thus, betweennozzles s1 and s2 within the section a-b there is provided a vacuum zonep1-. The development of this vacuum is assisted by a surface ap, locatedbetween nozzles s1 and s2 and facing section a-b. The vacuum zone can beused for sucking web W into the contact with dryer wire k2 over sectiona-b. Since the vacuum zone terminates upstream of point c, the transferof web W from one wire to another is facilitated.

The nozzle s2 directed in the traveling direction of second dryer wirek2 has two functions, namely serving as an ejector nozzle producingvacuum within a zone limited directly by section a-b of the wire as wellas an over-pressure within section c-d and especially in gap n+ betweendryer wire k2 and guide roll h2, for taking care of pocket ventilationas the blowing air finds its way into pocket t1,2.

In practice, the nozzles s1 and s2, vacuum surface ap, and air guidingsurface op can be simply formed such that the surfaces ap and op areprovided by the dryer wire k2 facing side of a common air-blowing box 3located inside the wire loop, whereby nozzle s2 opens on the sidebetween surfaces ap and op and nozzle s1 is located at the end of thebox upstream of point a, at which the wire and the web depart from dryercylinder 2.

Within the section e-f, wherein the dryer wire k2 progresses from guideroll h2 to cylinder 2 together with web W received thereon at guide rollh2, there is in turn created another vacuum zone p2- on the other sideof the dryer wire relative to pocket t2,1. This vacuum zone is producedby arranging an ejector surface ep inside the dryer wire loop oppositeto the wire. Between the ejector surface and the wire within this zonethere is induced a vacuum, since the wire has a tendency of grabbing airtherealong when traveling at a high speed. In order to intensify thevacuum, the ejector surface ep forms an angle with the wire which can bein the order of 0°-15°, opening slightly in the wire travelingdirection. The vacuum zone extends in the direction opposite to the wiretraveling direction all the way to gap n- between the jacket of guideroll h2 and the wire section e-f. In order to intensify the gap vacuum,the ejector surface ep extends in this direction all the way to the gap,for example beyond a plane tangential to the jacket of guide roll h2 andperpendicular to wire k2, wherein it is joined at point g by the archedsurface hp following the jacket of roll h2. As shown in the drawing, theejector surface ep is constituted of the wire section e-f facing side ofthe same air-blowing box, which also includes vacuum surface ap and airguiding surface op. The arched wall, connecting air guiding surface opand ejector surface ep and facing the jacket of guide roll h2, isprovided with the above-mentioned sealing t which, at the same time,separates gaps n+ and n- as well as the respective plenum and vacuumzones from each other.

Thus, a single air-blowing box located inside the dryer wire loop, bymeans of a proper design of its sides, can be used for producing boththe vacuum zone p1- for securing the adherence of web W to the wire oversection a-b, the plenum zone p1+, n+ taking care of the ventilation ofpocket t1,2, as well as the vacuum zone p2-, n- for securing theadherence of web W to dryer wire k2 or k1 upon its transfer from thezone of action of the suction-equipped guide roll h2 or h1 at point e tobe supported by the wire k2 or k1 within section e-f. In addition, itshould be appreciated that the plenum zone p1+, n+ for providing theventilation of pocket t1,2 or t2,1 also improves the adherence of web Wto the dryer wire within sections which define the corresponding pocketand are located immediately upstream and downstream of dryer cylinder 1or 2.

The edges of vacuum zones p1- and p2- can be provided with known edgeblows for preventing the flow of leakage air from the surroundings atthe wire edges into the vacuum zone.

The air-blowing box 3 conceivably comprises an air-blowing part 3ab,which receives blow ducts and is provided with nozzles s1 and s2, aswell as an air guiding part 3cf mounted thereon and including airguiding surfaces op and an ejector surface ep as well as surface hpconnecting them. Due to the configuration of a papermaking machine theseparts can sometimes be separate parts, which, when positionedsuccessively, provide the above assembly inside the wire loop.

Furthermore, the drawing illustrates how to arrange between the blowingpart and the air guiding part a flow regulator, such as a damper platesp, capable of regulating the amount of air flowing from nozzle s2through dryer wire k2 into pocket t1,2. The damper plate is located in aflow duct that connects the surface downstream of nozzle s2 with ambientair. When the damper plate is open, some of the amount of air enteringin the air-blowing part is allowed to flow out. When the damper plate isclosed, all the air flows from the nozzles which are in communicationwith the air-blowing part. In cross-wire direction, the box can befurther provided with a plurality of damper plates that can be adjustedfor affecting the distribution of air in lateral direction.

I claim:
 1. A method for ensuring the run of the web in themulti-cylinder dryer of a papermachine, including a first tier ofcylinders for passing therealong a web supported by a first dryer wireon the cylinders included in the first tier of cylinders, as well as asecond tier of cylinders for passing therealong the web supported by asecond dryer wire on the cylinders included in the second tier ofcylinders, wherein the first dryer wire travels around a first wireguide roll between the cylinders of the first tier of cylinders and thesecond dryer wire travels around a second wire guide roll between thecylinders of the second tier of cylinders such that, upon passing aroundthe first wire guide roll, said first dryer wire forms a loop having atleast one section, along which said first dryer wire is free of the weband which in the wire travelling direction is immediately upstream of apoint, at which said first dryer wire is received on the first wireguide roll, said method including the steps of:mounting a first nozzlealong a first dryer wire section between the cylinder of the first tierand the first wire guide roll inside the first dryer wire loop forblowing air through the free section of the first dryer wire forventilating a pocket formed between the free section and a cylinderincluded in the second tier of cylinders; forming a plenum zone betweenan air guiding surface located inside the wire loop downstream of saidfirst nozzle in the traveling direction of the first dryer wire, and thefree section; and blowing said air from said first nozzle into saidpocket through said plenum zone.
 2. A method according to claim 1,wherein said air is blown from said first nozzle upstream of an initialpoint of the free section as seen in the traveling direction of thefirst dryer wire.
 3. A method according to claim 2, wherein the firstdryer wire section between the dryer cylinder included in the first tierof cylinders and the first wire guide roll includes an initial section,along which said web travels upon the first dryer wire, wherein the webis transferred at the end of the initial section on the second guideroll of the second dryer wire, and wherein said method further includesproviding, in the first dryer wire traveling direction upstream of saidfirst nozzle, a second nozzle for blowing air against the travelingdirection of the first dryer wire for producing a vacuum zone delimitedby the first dryer wire between said first and second nozzles along thefirst part of the initial section.
 4. A method according to claim 3,wherein said plenum zone and said vacuum zone are created between afirst dryer wire facing side of an air-blowing box inside the first wireloop and the first dryer wire, and wherein said first nozzle is locatedon said side at the boundary of said vacuum zone and said plenum zone.5. A method according to claim 1, further including the step of sealingthe plenum zone between the air guiding surface and the first dryer wireat the end next to the first guide roll by means of a seal fittedbetween the guide roll jacket and a surface limiting the plenum zone. 6.A method according to claim 1, further including the step of providing asection of the first dryer wire located downstream of the first guideroll in the traveling direction of the first dryer wire between thefirst guide roll and a dryer cylinder included in the first tier ofcylinders inside the wire loop, with a vacuum zone directly limited bythe first dryer wire and an ejector surface facing said wire section. 7.A device for ensuring the run of the web in the multi-cylinder dryer ofa papermachine, including a first tier of cylinders including a firstdryer wire for supporting a web at and between the cylinders of thefirst tier of cylinders as well as a second tier of cylinders includinga second dryer wire for supporting the web at and between the cylindersof the second tier of cylinders, a first wire guide roll, around whichthe first dryer wire is adapted to travel between the cylinders of thefirst tier of cylinders and a second wire guide roll, around which thesecond dryer wire is adapted to travel between the cylinders of thesecond tier of cylinders, whereby said first dryer wire passing aroundthe first wire guide roll forms thereat a loop having at least onesection, along which the wire is free of the web and which in the webtraveling direction is immediately upstream of a point, at which thewire is received on the first wire guide roll, said device including:afirst nozzle mounted along a section of the wire between the cylinder ofthe first tier of cylinders and the first wire guide roll inside thedryer wire loop, said nozzle being directed such that the air blown byit travels through the free section of the first dryer wire forventilating a pocket formed between it and a cylinder of the second tierof cylinders, and in the traveling direction of the first dryer wireinside the wire loop, said first nozzle being followed by an air guidingsurface and a plenum zone formed between the air guiding surface and thefree section.
 8. A device according to claim 7, wherein, as seen in thetraveling direction of the first dryer wire, said first nozzle islocated upstream of an initial point of the free section.
 9. A deviceaccording to claim 8, wherein a section between the first dryer cylinderand the first wire guide roll comprises an initial section wherein theweb is adapted to travel on the first dryer wire, the end of the initialsection having a transfer point for transferring the web onto the seconddryer wire traveling on the cylinders of the second tier of cylinders,and wherein in the traveling direction of the first dryer wire upstreamof said first nozzle, there is a second nozzle directed against thetraveling direction of the first dryer wire and between the first andsecond nozzles a vacuum zone is formed limited by the first dryer wire.10. A device according to claim 9, further comprising an air-blowing boxlocated inside the wire loop of the first dryer wire, said plenum zoneand said vacuum zone being located between the side of said air-blowingbox facing the first dryer wire and the first dryer wire, and said firstnozzle being located on said side at the boundary of said plenum andvacuum zones.
 11. A device according to claim 7, wherein the plenum zonebetween the air guiding surface and the first dryer wire is limited atthe end next to said first guide roll by a seal fitted between the guideroll jacket and a surface limiting said plenum zone.
 12. A deviceaccording to claim 7, wherein an ejector surface is located inside thewire loop of the first dryer wire opposite to a section of the firstdryer wire located downstream of the first guide roll in the travelingdirection of the first dryer wire between the guide roll and a dryercylinder included in the first tier of cylinders, as well as a vacuumzone directly limited by the first dryer wire within said section insidethe wire loop.